Would you consider using the Order Page Attributes? You need to edit each Portfolio Items then add a number on “Order” attribute field. First, edit includes > admin > register-portfolio.php and find this code on line 34:

I am not sure how these characters are handled in other countries but here in Austria if we got an alphabetic list of items and there is an item with Ä,Ö or Ü those are directly located after A, O and U even though they are technically appended to the alphabet, if listed separately. I guess this is also how php queries interpret it, so I am afraid I have no idea how one would change that. As far as I get it this is neither a WordPress nor Theme issue but an issue of php and mysql itself :/

That’s what happening now, they are listed after A and O instead of last in the alphabet. The ordinary WP-sorting works, and it works in your older themes (at least in Choices and Eunoia). So this must something in the sorting in Enfold.

Enfold uses the default WP_Query class in /wp-content/themes/enfold/config-templatebuilder/avia-shortcodes/portfolio.php to query the portfolio entries. You can try to output the result of the query by replacing

$this->entries = new WP_Query( $query );

with

$this->entries = new WP_Query( $query );
print_r($this->entries);

If the $this->entries output lists the posts in the wrong order too it’s not a theme issue because then WP returns the posts in the wrong order.